1. Field of the Invention
The present invention relates to an obstacle detecting system which is suitable for a vehicle such as automobile. More particularly, it relates to an obstacle detecting system for detecting the distance from an obstacle and the relative velocity between the vehicle and the obstacle.
2. Description of the Prior Art
Recently, automobiles have been so widely used that traffic accidents have become commonplace. Accordingly, traffic accidents have become quite an important social problem. According to statistical data, most accidents are caused by driver error.
A need has developed for a safety device such as an automatic system for preventing collisions. In one safety device for preventing automobile accidents, knowledge of the distance to the obstacle and the relative velocity of approach must be ascertained. This is usually accomplished by radar. Heretofore, it has been known to employ a doppler radar for detecting relative velocity; a pulse radar and an AM phase difference radar for detecting distance; and a two frequency doppler radar for detecting distance and relative velocity.
However, these conventional radars have certain disadvantages which preclude their practical utilization. The doppler radar can be used for detecting relative velocity. However, it cannot be used for detecting distance. Accordingly, an obstacle too distant to be considered may be erroneously detected as an obstacle. Accordingly, the doppler radar, by itself, cannot be practically used. The pulse radar can be used for ascertaining distance. However, it is difficult for it to detect relative velocity. Accordingly, even though there may be no possibility of collision because of an absence of relative velocity, a rather near obstacle may be erroneously detected. Accordingly, the pulse radar, by itself, cannot be practically used. The AM phase difference radar, the FM phase difference radar and the two frequency doppler radar have the disadvantage of not providing sufficient distance information where a plurality of obstacles is involved. When there is a first obstacle present within a dangerous distance along with a second obstacle having higher reflectivity at a distance too far to be considered dangerous, the radar erroneously indicates the presence of no dangerous obstacle. The two frequency doppler is used to find distance by a phase difference formed by two transmission frequencies whereby the distance can be ascertained by a simple circuit such as may be used in a small radar for an automobile. However, when the doppler indicating relative velocity is zero or rather small, it is difficult to ascertain distance.
In order to overcome these disadvantages, it has been suggested that the doppler and the pulse radar be combined. By combining the doppler radar and the pulse radar, relative velocity can be generated by the doppler radar and distance information can be generated by the pulse radar. Where a plurality of obstacles is involved, it is possible to omit information of the distant obstacle by employing a range gate. The pulse radar can detect distance even though no relative velocity is found. However, the employment of two kinds of radars is quite expensive with the result that it is not practical.